Late Pleistocene Depositional Conditions In Ejina Basin,China,Revealed By GN200 Sediment Core

The northwestern arid area of China(NAAC)is the most important dust source region in China.It yields nearly half of the world's annual dust flux.This dust has been transported to the populated eastern regions of China and beyond.Furthermore the Ejina Basin in northwestern China serves as an important sediment sink for material transport from the surrounding mountain belts.Both processes have been affecting the environment in the arid region to a large extent.Hence,the arid region has become a hot spot in research.Due to the endorheic character of the Ejina Basin it is thought as a good geological archive of climate and environmental change.The sediments in the Ejina Basin were mainly transported by the Heihe River system during the Quaternary period.A 224 m long sediment core down to the pre-Quaternary was obtained from the center of the Ejina Basin.It provided a complete sequence of sediments which are useful to reconstruct environmental and climatic changes.In this research the upper 65 meter of core GN200 was analyzed by means of grain size composition,geochemistry(organic matter and carbonate)and end-member modeling approaches to identify sedimentary processes.According to the chronology,sediment depositional processes refer to three units,comprising the time from the MIS3 to the present.During MIS 3 from 65m to 35m sediment depth,the dominant processes could be attributed to lacustrine and semi-lacustrine deposition,accompanied by eolian processes.The landform was characterized as a playa-like environment,consisting of many lakes,ponds and swamps that covered vast areas of the desert.Eolian sands in the sediment core however,indicate that the area was still capable to enable eolian transport under still arid climatic conditions.Water resources for maintaining lacustrine and semi-lacustrine environment may have originated from the upper catchment.During MIS 2 from 35?11 m core depth,the environment was strongly influenced by enhanced eolian processes,while lakes and swamps disappeared to a large extent.The environment was cold and dry,while water supply was reduced and possibly captured in glacier ice of the upper catchments.The Holocene from 11m to the top of core GN200 was dominated by fluvial processes under probably warm and wet conditions at that time.The gravels on the top 3 m of the core are characteristic of a very high energy flow that happened during the early to mid-Holocene time.The coarse material suggests local transport processes-rather than long distant transport from the Qilian Mountains.Our research suggests that the water discharge from the Tibetan Plateau was the dominant factor which affected the evolution of landform and environment.Our results also show relatively dry conditions that prevailed during the MIS 2 and MIS 3,maybe similar to the present,but a large lake area during MIS 3 as previously suggested could not be confirmed by this record.During the early Holocene dramatic climate changes may have occurred.They were associated with high water input that enabled the local wide-spread transport of gravels which nowadays form the Gobi plain.